Curable silicone compositions are applied to paper substrates to aid in the release of adhesive materials thereon. Laminates comprising a release paper coated with a pressure sensitive adhesive and a sheet material that can be a decorative lamina or label are used by stripping off the release liner which is discarded and affixing the lamina or label onto a surface.
Typically these release compositions cure by one of two mechanisms, thermal curing or photo-catalytic curing. Thermally curing paper release systems generally are comprised of the following alkenyl siloxane based compositions:
(A) a linear alkenyl substituted polysiloxane polymer that is the primary component or base polymer of the curable composition; PA1 (B) an hydrosilylation addition cure catalyst, typically either a platinum or rhodium based catalyst; PA1 (C) a cure inhibiting compound or mixtures thereof to increase the useful life of the coating bath; and PA1 (D) a hydride functional cross-linking silicone, typically a methyl hydrogen siloxane polymer, copolymer or oligomer. PA1 (A') a substantially linear vinyl chainstopped polymer; PA1 (B') a linear methyl hydrogen polymer; PA1 (C') a methyl vinyl polysiloxane having at least three vinyl groups per molecule; PA1 (D') a methyl hydrogen polysiloxane having at least three hydride hydrogen atoms per molecule; and PA1 (E') a platinum hydrosilylation catalyst. PA1 1) a linear alkenyl chainstopped polymer: EQU M.sup.vi D.sub.x M.sup.vi 4) PA1 2) multi-functional alkenyl copolymers: EQU M.sup.vi D.sub.x D.sup.vi.sub.y M.sup.vi 5) PA1 (A) a silanol stopped polysiloxane polymer that is the primary component or base polymer of the curable composition; PA1 (B) a condensation cure catalyst, almost always a tin containing condensation cure catalyst; PA1 (C) optionally a cure inhibiting compound or mixtures thereof to increase the useful life of the coating bath; and PA1 (D) a hydride functional cross-linking silicone, typically a methyl hydrogen siloxane polymer, copolymer or oligomer. PA1 a) an aqueous curable silicone emulsion, and PA1 b) an aqueous curable polyurethane emulsion PA1 a) an aqueous curable silicone emulsion, and PA1 b) an aqueous curable polyurethane emulsion PA1 a) a curable organopolysiloxane composition which may be a mixture of organopolysiloxanes comprising: PA1 b) an emulsifying agent selected from the group consisting of polyvinyl alcohol, non-ionic surfactants and mixtures thereof; PA1 c) water; and PA1 d) optionally additional reagents selected from the group consisting of wetting agents, biocides and coalescing agents. PA1 a) a polyisocyanate having at least two isocyanate (--NCO) functionalities per molecule. Suitable polyisocyanates include diisocyanate monomers, oligomers cited in the following. There are aliphatic polyisocyanates such as 1,6-hexamethylene diisocyanate (HMDI) and its isocyanurate-containing derivatives; cycloaliphatic polyisocyanates such as 4,4'-mthylene bis(cyclohexyl isocyanate) (H.sub.12 MDI), cyclohexane 1,4-diisocyanate and its isocyanurate derivatives; aromatic polyisocyanates such as 4,4'-diphenylmethane diisocyanate (MDI), xylyene diisocyanate (XDI), toluene diisocyanate (TDI), isophorone diisocyanate (IPDI), 1,5-naphthalene diisocyanate (NDI), 4,4',4"-triphenylmethane diisocyanate, and their isocyanurate-containing derivatives. Mixtures or the reaction products of polyisocyanates can be used. Polyisocyanates contain the reaction products of these diisocyanate including isocyanurate, urea, allophanate, biuret, carbodiimide, and uretonimine entities are also included. PA1 b) a polyol having at least two hydroxy functionalities per molecule and a molecular weight ranging from 250 to 5000 g/mole. The polyol may be selected from those commonly found in polyurethane manufacturing. They include hydroxy-containing or terminated polyesters, polyethers, polycarbonates, polythioethers, polyolefins, and polyesteramides. Suitable polyester polyols include hydroxy-terminated reaction products of ethylene glycol, propylene glycol, diethylene glycol, neopentyl glycol, 1,4-butanediol, furan dimethanol, polyether diols, or mixtures thereof, with dicarboxylic acids or their ester-forming derivatives. Polyesters obtained by the polymerization of lactones, such as caprolactone may also be used. PA1 c) a hydrophilic group containing compound containing either two isocyanate or isocyanate-reactive groups per molecule. The hydrophilic group may be of ionic, ionic precursor or nonionic type. The isocyanate-reactive group include those compounds containing active hydrogen such as diols, polyols, diamines, and polyamines. PA1 d) a neutralizing compound for the hydrophilic group in c). Diamine or triamine such as diethyl amine or triethylamine is effective in neutralizing carboxylic group and yields a neutralized anionic hydrophilic site on the polyurethane. PA1 e) a chain extender that reacts with the excess or available isocyanate groups in the presence of aqueous medium and leads to a high molecular weight polyurethane aqueous dispersion.
While the general practice usually employs linear base polymers, (A), solventless, high solids content formulations have been described. As described in U.S. Pat. No. 4,448,815 ('815) a linear alkenyl siloxane base copolymer is a copolymer of: EQU (1) R.sub.c R.sub.d.sup.1 Si.sub.(4-c-d) /2 1)
where R is generally an alkyl radical, R.sup.1 is a low molecular weight olefinic substituent such as vinyl or allyl, c has value from 0 to 2 and the average of value of the sum c+d is 0.8 to 3; and EQU (2) R.sub.n SiO.sub.(4-n)/2 2)
where R is generally an alkyl radical and n has a value of 0.8 to 2.5. The preferred base copolymer of the '815 patent has the following linear structure: EQU (H.sub.2 C.dbd.CH)R.sub.2 Si--O--(R.sub.2 Si--O--).sub.i --(RR.sup.1 Si--O--).sub.j --SiR.sub.2 (H.sub.2 C.dbd.CH)
where the subscripts i and j are integers.
U.S. Pat. No. 4,774,111 ('111) describes a variation of the above linear copolymer where the R group in formula 2 is selected from alkyl and alkenyl radicals. The polymer of the '111 patent is defined as being substantially linear, i.e. having no more than a trace amount of T or Q groups. This substantially linear requirement for alkenyl functional heat curing silicone release compositions is repeated in U.S. Pat. Nos. 4,772,515; 4,783,552 and 5,036,117.
In contrast, the possibility of branched alkenyl polymers is admitted by the structural formulas recited in U.S. Pat. No. 4,057,596 ('596). In the '596 patent the composition comprises:
Component (C') is described in the '596 patent as containing (H.sub.2 C.dbd.CH)(CH.sub.3)SiO.sub.2/2 (D.sup.vi), (H.sub.2 C.dbd.CH)(CH.sub.3).sub.2 SiO.sub.1/2 (M.sup.vi), and (H.sub.2 C.dbd.CH)SiO.sub.3/2 (T.sup.vi), units either singly or in combination with (CH.sub.3).sub.2 SiO.sub.2/2 (D), (CH.sub.3).sub.3 SiO.sub.1/2 (M), and (CH.sub.3)SiO.sub.3/2 (T). The optional inclusion of vinyl substituted T units and methyl T units permits the composition of the '596 patent to possess branched structures.
U.S. Pat. No. 4,386,135 describes a terminally unsaturated silicone polymer having the formula EQU R.sub.4-a Si((R.sub.2 SiO--).sub.b OSiR.sub.2 R.sup.2).sub.a 3)
where a may be 2, 3, or 4. When a=4 the formula produces a Q resin. When a=3, a T structure results and the structure possesses only a single branch point. When a=2, the formula devolves to an alkenyl chain stopped linear polymer.
The alkenyl functional silicone polymer paper release compositions typically used fall into one of two categories:
where M.sup.vi indicates an alkenyl chainstopping M group or
where D.sup.vi indicates an alkenyl substituted D group. The alkenyl chain stopped polymers, M.sup.vi D.sub.x M.sup.v, generally cure faster than the multi-functional copolymers, M.sup.vi D.sub.x D.sup.vi.sub.y M.sup.vi. As paper release composites are delaminated, the formulations based on the linear alkenyl chainstopped polymers show significant increases in the delamination force necessary as delamination speed increases. In contrast, while the multi-functional alkenyl polymers tend to have a slower curing speed the increase in delamination force with increasing delamination speed is not nearly as great proportionately.
An older technology for paper release, largely supplanted by the thermal curing alkenyl based compositions and radiation curable photo-catalytic or free radical acrylate or epoxy compositions is based on condensation curable systems. Such condensation curable systems utilize a silanol stopped siloxane in place of the alkenyl functional siloxane. Thus such compositions typically are composed of:
While the alkenyl based compositions may have the alkenyl functionality anywhere in the alkenyl siloxane, a significant disadvantage of the silanol based condensation curable release formulations has been that the polymer is almost always silanol stopped.
Either of these two formulations has generally been satisfactory from the standpoint of printing graphics or text on the substrate prior to coating with the releases formulation. These formulations have thus received wide application in the preparation of various types of labels. The surface coating, which provides the desired release characteristic, however, usually does not permit the subsequent application of additional graphics or text, i.e. it has not been generally possible to print additional text or graphics onto the surface of the release coated substrate. While most applications of this technology are not unduly limited by this consideration, the manufacture of single use security instruments such as linerless postage stamps can not be accomplished with the state of this technology because cancellation inks do not adhere to the surface. Thus the development of a printable release coating would enable extension of release coating technology to the preparation of postage stamps which could be manufactured in the same fashion as other labels but would accept cancellation inks. Further, the technology would enable overprinting of conventional labels with identifying data such as production number, date produced, and the like, which information must currently be printed underneath the release coating on the substrate. A printable release coating is a sought after improvement in release coating technology.